The discrepancy between manufacturers’ claims and the road data was especially stark for vehicles with smaller engines, which generally have to work harder to accelerate.

Tests showed that vehicles with an engine size up to one litre had an average advertised 60.3mpg, but consumption was measured at 38.6mpg in tests, a drop of 36 per cent.

Average consumption for cars with one to two-litre engines was measured at 46.7mpg, 21 per cent lower than the advertised 59.1mpg. This meant they travelled further on the same amount of fuel than the average smaller car. . . .

It is also interesting that there is almost no difference in fuel economy between midsize and larger cars.There are at least a couple US cars with 1 Liter engines.2014 Ford Fiesta 1.0L EcoBoost2014 Smart fortwoUPDATE: Here is something that I found at Green Car Reports:

Simple to explain from a human behavior perspective. IC engines have a fuel efficiency optimum power range, which isn't full throttle.Human drivers have a "minimum tolerable acceleration" requirement when driving.Thus on little engines, they boot it more.Also, they are likely psychologically encouraged to do so:Thought process of "well, its a small efficient car, so flooring it probably doesn't burn that much gas"